The goal of this project is to investigate factors predicting a couples natural fertility, as well as their response to environmental exposures. The emphasis is on applying innovative statistical methods to detailed data on the timing of menstrual bleeding, intercourse, and ovulation collected prospectively in non-contracepting menstrual cycles. Surprisingly little is known about the basics of human reproduction, such as the relationship between male and female age and fecundity. We have utilized data from Allen Wilcox's North Carolina Early Pregnancy Study in the past, and have recently focused on data from a multinational European fecundability study of users of natural family planning methods. Work has progressed in several areas: (1) the development of improved methods for assessing factors predictive of the day-specific probabilities of conception in the menstrual cycle; (2) the application of these methods to investigate the impact of cervical mucus and age; and (3) the development of methods for analysis of menstrual cycle data on hormones and cervical mucus. Using the European data base and a new Italian data base, we have demonstrated that cervical mucus secretions play a prominent role in the regulation of the fertile interval and the day-specific probabilities of conception. We hypothesize that sperm with lowered motility, either due to the natural process of male aging or to environmental exposure, may be less capable of survival and transport in the female reproductive tract unless high levels of estrogenic-type mucus are present. A striking interaction between mucus and the association between male age and fertility provides support for this hypothesis.